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Abstract:

A latent heat storage material is formed of at least two plies of a
compressible graphitic material in which graphite wafers are arranged
substantially in layer planes lying one on the other and which is
infiltrated with at least one phase change material. The surface of each
ply is provided with a structuring reaching the outsides of the graphite
material bundle. The evacuation and infiltration travel lengths in the
layer planes, due to the structuring, amounts to a maximum of 200 mm.

Claims:

1. A graphite matrix body for a latent heat storage material, comprising:
at least two plies of a compressible graphite material with graphite
platelets disposed substantially in layer planes lying one above the
other; each ply of said graphite material having a surface formed with a
surface structuring reaching to a marginal surface thereof and thereby
reaching to an outside of the graphite matrix body for defining
infiltration channels and evacuation channels among said plies of
graphite material for phase change material; said graphite material being
configured to absorb an amount of phase change material having a mass
exceeding a mass of said graphite material, wherein the phase change
material enters the graphite matrix body through said infiltration
channels and infiltrates said graphite material from said infiltration
channels.

2. The graphite matrix body according to claim 1, wherein the mass of
phase change material impregnated in said graphite material is at least
twice the mass of said graphite material.

3. The graphite matrix body according to claim 1, wherein the mass of
phase change material impregnated in said graphite material is at least
three times the mass of said graphite material.

4. The graphite matrix body according to claim 1, wherein a travel length
of said evacuation and infiltration travel paths in said layer planes due
to said structuring amount to a maximum of 200 mm.

5. The graphite matrix body according to claim 4, wherein said travel
lengths of said evacuation and infiltration travel paths in the layer
planes due to the structuring amount to a maximum of 50 mm.

6. The graphite matrix body according to claim 1, wherein said
structuring is in the form of channels having a ratio of a depth to a
width in a range of 20:1 to 1:20.

7. The graphite matrix body according to claim 6, wherein said channels
are arranged parallel to said graphite layers.

8. The graphite matrix body according to claim 6, wherein said channels
are arranged in a configuration selected from the group consisting of a
rectilinear configuration, a meander-shaped configuration, or a
herringbone shape configuration.

9. A latent heat storage material, comprising: a bundle formed of two or
more plies of a compressible graphitic material with graphite wafers
disposed in layer planes lying one above the other, said bundle having an
exterior and an interior; said plies having surfaces formed with
structuring defining evacuation and infiltration paths extending from the
interior to the exterior of said bundle, a travel length of said
evacuation and infiltration paths in the layer planes amounting to no
more than 200 mm; and an amount of phase change material infiltrated in
said compressible graphitic material.

10. A method of producing a latent heat storage material, which
comprises: providing a plurality of plies of a compressible graphitic
material, and providing up to 30% of a surface of each ply with a
structuring reaching the outsides of the material; bringing two or more
plies of the compressible graphitic material into contact with one
another, and pressing the plies formed with the structuring at a
temperature of up to 400.degree. C. and at a pressure of between 0.1 MPa
and 200 MPa.

11. The method according to claim 10, which comprises evacuating the
graphite material and infiltrating the layer material with phase change
material in one direction or from one side.

12. The method according to claim 10, which comprises pressing or rolling
channels into the plies of compressible material, the channels having a
cross section with sharp edges.

13. The method according to claim 10, which comprises milling channels
into the material.

[0002] The invention relates to a latent heat storage material which
consists of at least two plies of a compressible graphitic material and
is infiltrated with at least one phase change material and to a method
for producing such a latent heat storage material.

[0003] Latent heat storage materials based on graphitic materials which
are mixed, impregnated or infiltrated with a phase change material are
known from the German published patent application DE 196 30 073 and from
European patent application EP 1 598 406. The graphitic materials form a
highly heat-conductive matrix for the substantially less heat-conductive
phase change materials and therefore allow a better heat exchange of the
latent heat storage materials thus obtained. In particular, for the
production of simple moldings, the pressing of expanded graphite that is
precompacted into boards is appropriate. Infiltration of moldings
consisting of compacted expanded graphite is impeded by the low rate of
penetration of phase change material. For such boards consisting of
compacted expanded graphite, long process times for the evacuation and
infiltration are necessary in order to avoid the situation where too
little PCM is taken up. Disadvantageous here are long process times or a
low storability or storage capacity of the latent heat storage material
thus produced.

BRIEF SUMMARY OF THE INVENTION

[0004] The set object of the invention is to specify a latent heat storage
material and a corresponding structural device which includes at least
two plies of a compressible graphitic material and is infiltrated with at
least one phase change material. The set object of the invention,
furthermore, is to provide a method for producing such a latent heat
storage material.

[0005] With the above and other objects in view there is provided, in
accordance with the invention, a graphite matrix body for latent heat
storage material, comprising:

[0006] at least two plies of a compressible graphitic material with
graphite platelets disposed substantially in layer planes lying one above
the other and infiltrated with at least one phase change material;

[0007] each ply having a surface formed with a surface structuring
reaching the outsides of said graphite material and defining evacuation
and infiltration travel paths; and

[0008] a travel length of said evacuation and infiltration travel paths in
said layer planes due to said structuring amounting to a maximum of 200
mm.

[0009] In accordance with an added feature of the invention, the travel
lengths of the evacuation and infiltration travel paths in the layer
planes due to the structuring amount to a maximum of 50 mm.

[0010] In other words, the objects of the invention are achieved with
proposed structuring. The structuring promotes the evacuation of the
graphite matrix and also the infiltration of the package with phase
change material (PCM). Further, it improves the access to the PCM in the
finished assembly and increases the corresponding amount of PCM and
raises the capacity of the device. As a result of the structuring, the
air included in the graphite matrix is removed more quickly and more
completely and a faster infiltration of the graphite matrix and also a
higher degree of filling with the phase change material are achieved.

[0011] The terms "infiltration" and "impregnation" as understood herein
refer to molecular adhesion processes and to microscopic, capillary
activity. Infiltration means that a material (i.e., PCM) permeates
something (i.e., the graphite bulk) by penetrating its pores and
interstices. The PCM which is infiltrated into the graphite matrix does
not substantially alter the graphite matrix, but is rather "stored" in
the interstices formed between the graphite platelet layers and inside
the graphitic molecular structure. The process may also be referred to as
saturation, where the PCM "saturates" the graphite matrix and the
structural form and shape of the saturated graphite bundle remains
substantially unchanged relative to the graphite bundle prior to its
infiltration and impregnation with the PCM.

[0012] As best understood, the graphite material forms the structure of
the matrix and, at the same time, acts as the primary thermal conductor.
The fact that the heat transport paths into and out of the latent heat
storage device are provided by the graphite matrix walls themselves,
enables substantially the entire amount of the PCM (i.e., the heat
storage material itself) to react, and to react quickly, to the
introduction of heat content or to the extraction of heat content. During
the introduction of heat, the PCM acts as a heat sink while it acts as a
heat source during the extraction of heat.

[0013] The compressible graphite material used for improving the thermal
conductivity of the latent heat storage material is produced in a way
that is known per se by the thermal expansion of interstitial graphite
compounds into so-called expanded graphite and by the subsequent
compression of the expanded graphite into flexible sheets or into boards.
Reference is had, for example, to U.S. Pat. No. 3,404,061, to German
patent DE 26 08 866, and to U.S. Pat. No. 4,091,083, which are
incorporated by reference herein.

[0014] The compressible graphite plies may already have the bulk density
which is intended for them in the finished latent heat storage material.
The pressure force applied when the plies of compressible graphite are
pressed together to produce the latent heat storage material shall then
not exceed the compression pressure required for achieving the given bulk
density of the compressible graphite ply. However, even initially
compressible graphite plies with a lower bulk density from the final bulk
density in the finish-pressed latent heat storage material may be
applied. Only then is the intended final bulk density generated when the
components of the latent heat storage material are pressed together.

[0015] The groove depth in the rough-pressed article should preferably
amount to at least 3.5 mm. The pressing of the rough-pressed articles
into bundles, first in height and then in bundle width, does not result
in a homogeneous degree of pressing of the strips. The degree of
cross-linking of the strips decreases in the pressing direction and
opposite to the pressing direction leads to ever smaller groove depths.

[0016] Pressing should preferably take place in the order that the bundles
are first pressed width-wise and then height-wise. In this case, a height
of 12.2+/-0.2 mm and a width of 30.7+/-0.2 mm of the rough-pressed
articles, with grooves which are approximately 3.5 mm deep and
approximately 4.5 mm wide, have proved to be advantageous. Preferably,
30-250 strips of the rough-pressed articles are pressed into a bundle.

[0017] As the individual strips are layered in plies, they are placed so
that the surface structuring reaches the outsides of the pressed graphite
material bundle. This, therefore, defining evacuation channels and
infiltration channels. As a guide, the travel length of the evacuation
and infiltration travel paths in the layer planes due to the structuring
amounts to a maximum of 200 mm and, preferably, to no more than 50 mm.

[0018] The channels forming the surface structuring may be pressed or
rolled into the plies of compressible material and the channels are
preferably formed to have a cross section with sharp edges. In the
alternative, the channels may be milled into the material. It is possible
to provide individual strips that are then layered into a multilayer
bulk. The strips may thereby be formed with the surface structuring
(e.g., channels) prior to layering, or they may be placed to form a layer
ply of the bulk and then the channels may be formed in each such layer
before the next layer is placed on top.

[0019] The structuring is preferably in the form of channels formed in the
surface of the ply material and having a ratio of depth to width in a
range of 20:1 to 1:20. As the channels are formed on the surfaces of the
plies, or the layer strips, the channels are arranged parallel to the
graphite layers in the layered bulk.

[0020] The channels may be arranged in a variety of configurations, such
as rectilinear, meandering, or a herringbone shape configuration. The
channels are preferably arranged to extend in an evacuation and/or
infiltration direction.

[0021] In a preferred process sequence, there are first provided a
plurality of plies of a compressible graphitic material (e.g., expanded
graphite). Up to 30% of a surface of each ply is provided with a
structuring that reaches to the outsides of the material. Then two or
more plies of the compressible graphitic material are placed in contact
with one another, and the layered plies formed with the structuring are
compressed at a temperature of up to 400° C. and at a pressure of
between 0.1 MPa and 200 MPa.

[0022] Then the compressed bulk of expanded graphite is evacuated and
infiltrated with phase change material. The evacuation and the
infiltration may be effected in one direction or from one side.

[0023] Other features which are considered as characteristic for the
invention are set forth in the appended claims.

[0024] Although the invention is described herein as embodied in a latent
heat storage material and a production method, it is nevertheless not
intended to be limited to the details shown, since various modifications
and structural changes may be made therein without departing from the
spirit of the invention and within the scope and range of equivalents of
the claims.

[0025] The construction and method of operation of the invention, however,
together with additional objects and advantages thereof will be best
understood from the following description of the exemplary figures and of
specific examples and comparative examples.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0026] FIG. 1 is a perspective view of a strip of expanded graphite formed
into a shape according to the invention;

[0027] FIG. 2A is a partial top view of the strip shown in FIG. 1;

[0028] FIG. 2B is a partial top view of an alternative embodiment;

[0029] FIG. 2C is a partial top view of yet another alternative
embodiment;

[0030] FIG. 3 is a perspective view of a bundle of strips according to
FIGS. 1 and 2A partially assembled; and

[0031] FIG. 4 is a perspective view of an exemplary bundle assembled from
the strips according to FIG. 2B.

DESCRIPTION OF THE INVENTION

[0032] Referring now to the figures of the drawing in detail and first,
particularly, to FIG. 1 thereof, there is shown an exemplary strip 1
formed by pressing expanded graphite. The strip 1 is formed with a
central groove 2 which extends along its entire length in the center of
the upper flat surface 3 and in the center of the lower flat surface 4. A
graphite platelet alignment which is approximately 45° between the
lower and upper surfaces 3, 4 is indicated on the side wall 5.

[0033] The dimensions of the strip are driven by the respective
requirements posed of the resulting phase change material device. Here,
the strip 1 has a length of approximately 50 centimeters (1/2 m), a width
of approximately 4 centimeters, and a thickness of approximately 1.5
centimeters. A great variety of other dimensions are available. However,
it is paramount that proper and efficient impregnation/infiltration of
the strip is assured. Accordingly, the dimensions of the groove 2 (for
efficient delivery of the PCM into the graphite and efficient heat
exchange delivery) and also the distance of the groove from the remaining
material are taken into account in selecting the dimensions.

[0034] FIG. 2B illustrates an alternative embodiment in which the grooves
2' traverse the top surface 3 and the bottom surface 4 at an angle of
45° relative to the longitudinal extent of the strip.

[0035] FIG. 2C illustrates an alternative embodiment in which the grooves
2'' in the top surface 3 and the bottom surface 4 form a fishbone
pattern. Many other designs are available, depending on the functional
and structural requirements of the device.

[0036] With reference to FIG. 3, the individual strips 1 may be stacked
into a bundle 6, with the strips 1 back-to-back so that the grooves 2 of
adjoining strips 1 form flow channels for PCM into and out of the bundle
6.

[0037] FIG. 4 illustrates one of many further alternatives. Here, the
strips 1 of FIG. 2B are stacked on one another. In addition,
alternatively placed strips are offset from one another in the
longitudinal direction by one half the spacing between the individual
grooves 2'. This placement provides for a multitude of delivery channels
that are relatively densely distributed about the bundle 6.

[0038] The invention will now be explained by way of a plurality of
examples in which the inventive concept was implemented.

Comparative Example 1

[0039] Strips of expanded graphite with the dimensions 480 mm length, 40
mm width, 15 mm thickness were layered and pressed into a bundle. The
resulting bundle weight of the compacted graphite amounted to 862 g. The
bundle was introduced into a bag and evacuated with the aid of a vacuum
pump. The evacuation was driven to a subatmospheric pressure of 10 mbar.
The evacuation time amounted to 220 s.

[0040] Infiltration with 3100 ml of water as phase change material
subsequently took place. After storage for approximately six hours,
approximately 300-400 ml of free water was still found. That is,
approximately 2700-2800 ml of water infiltrated into a bundle of 862 g of
compacted, expanded graphite.

Comparative Example 2

[0041] In a similar way to example 1, a lighter bundle was assembled and
pressed together. The bundle weight of the graphite amounted to 770 g.
After an evacuation time of 220 s, infiltration with 3100 ml of water
took place.

[0042] Here, the deformation of the bag (i.e., bladder) was quite
pronounced. Sensor inoperative. Filling operation was concluded. The
amount of free water remained quite large. After storage for
approximately six hours, the remaining free water amounted to
approximately 500-600 ml.

Comparative Example 3

[0043] In a similar way to example 1, a bundle was assembled and pressed
together. The bundle weight of graphite amounted to 757 g. Here, the
evacuation time was increased to 500 seconds.

[0044] The filling operation with water took place approximately normally.
Deformation of the bag was slightly greater. The bundle was firm after
storage for 10 minutes. That is, the entire amount of water was
infiltrated in the graphite matrix

Example 1

[0045] Approximately 15 diagonal grooves were introduced by hand on each
of the two sides of the strips at an angle of approximately 45°
relative to the longitudinal extent. The bundle weight of the graphite
was 775 g. The compressed bundle was evacuated for an evacuation time of
500 s.

[0046] The filling operation proceeded normally. Deformation of the bag
was normal. The bundle was firm immediately. In other words, the water
(i.e., PCM) entered the graphite matrix substantially immediately,
without first forming a water pool in the bag.

Example 2

[0047] Before pressing, two longitudinal grooves and four diagonal grooves
were introduced on one side. The bundle weight of the graphite was 806 g
and the evacuation time was set to 220 s.

[0048] The filling operation proceeded normally. The deformation of the
bag was normal. The bundle was firm in the machine.

Example 3

[0049] Two longitudinal grooves were introduced on one side by hand in
series strips and the evacuation time was shortened. The bundle weight of
graphite was 780 g and the evacuation time was set to 90 s.

[0050] The filling operation proceeded normally. The deformation of the
bag was normal. The bundle was firm after storage of 10 minutes.

[0051] The results of further examples are illustrated in summary in Table
1.